Channel Estimation and Interference Cancellation for Virtual MIMO Demodulation

ABSTRACT

A method and system for wireless communication in a wireless communication network. The wireless communication network has a first mobile terminal and a second mobile terminal arranged in virtual multiple input, multiple output (“V-MIMO”) communication with a base station. A first wireless communication uplink channel corresponding to the first mobile terminal is estimated. The estimate is based on a first reference symbol signal and the cancellation of interference from a second reference symbol signal received from the second mobile terminal. A second wireless communication uplink channel corresponding to the second mobile terminal is estimated. The estimate is based on the second reference symbol signal and the cancellation of interference from the first reference symbol signal received from the first mobile terminal. The estimated first wireless communication uplink channel is used to demodulate a first data signal received from the first wireless device, and the estimated second wireless communication uplink channel is used to demodulate a second data signal received from the second wireless device.

FIELD OF THE INVENTION

The present invention relates to wireless communications and inparticular to a method and system for wireless communication channelestimation and interference cancellation used to demodulate virtualmultiple input, multiple output (“V-MIMO”) signals.

BACKGROUND OF THE INVENTION

Demand for high speed wireless communications is increasing at a fastpace. This is driven both by the sheer number of wireless communicationterminals being activated as well as the increasing bandwidth demand.The latter is in turn driven by the increasing number of applicationsconsuming the bandwidth, e.g., streaming multimedia, web browsing, GPSfunctionality, etc. As the computation capacity of the wirelesscommunication terminals increases, so too do the terminals' ability toexecute complex, bandwidth consuming applications.

Wireless communication networks, such as cellular networks, operate bysharing resources among the mobile terminals operating in thecommunication network. As part of the sharing process, resourcesrelating to assigned channels, codes, etc. are allocated by one or morecontrolling devices within the system. Certain types of wirelesscommunication networks, e.g., orthogonal frequency division multiplexed(“OFDM”) networks, are used to support cell-based high speed servicessuch as those under certain standards such as the 3rd GenerationPartnership Project (“3GPP”) e.g., Long Term Evolution (“LTE”), 3GPP2,e.g., Ultra-Mobile Broadband (“UMB”) and the IEEE 802.16 broadbandwireless standards. The IEEE 802.16 standards are often referred to asWiMAX or less commonly as WirelessMAN or the Air Interface Standard.

OFDM technology uses a channelized approach and divides a wirelesscommunication channel into many sub-channels which can be used bymultiple mobile terminals at the same time. These sub-channels and hencethe mobile terminals can be subject to interference from adjacent cellsand other mobile terminals because neighboring base stations and mobileterminals can use the same time and frequency resource blocks. Theresult is that spectral efficiency is reduced, thereby reducing bothcommunication throughput as well as the quantity of mobile terminalsthat can be supported in the network.

This problem is further exacerbated in multiple input, multiple output(“MIMO”) environments. Multiple Input, Multiple Output OrthogonalFrequency Division Multiplexing (“MIMO-OFDM”) is an OFDM technology thatuses multiple antennas to transmit and receive radio signals. MIMO-OFDMallows service providers to deploy wireless broadband systems that takeadvantage of the multi-path properties of environments using basestation antennas that do not necessarily have line of sightcommunications with the mobile terminal.

MIMO systems use multiple transmit and receive antennas tosimultaneously transmit data, in small pieces to the receiver, whichprocesses the separate data transmissions and puts them back together.This process, called spatial multiplexing, can be used to proportionallyboost the data-transmission speed by a factor equal to the smaller ofthe number of transmitting and receiving antennas. In addition, sinceall data is transmitted both in the same frequency band and withseparate spatial signatures, this technique utilizes spectrum veryefficiently.

MIMO operation implements a channel matrix (N×M) where N is the numberof transmit antennas and M is the number of receive antennas to definethe coding and modulation matrix for the wireless communication channelas a whole. The less correlated each column in the matrix is, the lessinterference experienced in each channel (as a result of the multipleantennas). In the case where there is a totally uncorrelatedarrangement, i.e., the dot product between columns is zero, the channelsare considered orthogonal to one another. Orthogonality provides theleast antenna-to-antenna interference, thereby maximizing channelcapacity, and data rate due to the higher post-processing signal tointerference and noise ratio (“PP-SINR”). PP-SINR is the SINR after theMIMO decoding stage.

Virtual MIMO (“V-MIMO”), also referred to as Multi-User MIMO (“MU-MIMO”)implements the MIMO technique described above by using multiplesimultaneously transmitting mobile terminals each having one or moreantennas. The serving base station includes multiple antennas. Althoughthe base station can treat virtual MIMO operation as traditional MIMO inwhich a single mobile terminal has multiple antennas and can separateand decode the transmissions from the multiple simultaneouslytransmitting mobile terminals, channel correlation among mobileterminals as discussed above results in channel capacity loss due tointer-mobile terminal interference.

Because wireless communication channels are subject to interference anddistortion, techniques have been developed to estimate certainproperties of the channel so that the receiver, e.g., base station, cantake these properties into account when decoding the received data. Forexample, multipath distortion and fading can alter the amplitude andphase of the transmitted wireless signal. The result is that, if thewireless communication channel is not accurately estimated, the decodeddata will likely be improperly decoded. For example, a 16QAM or 64QAM(quadrature amplitude modulation) signal modulates a plurality of bits.Decoding of those bits is based on the amplitude and phase of thereceived signal as applied to a modulation constellation. If theamplitude and/or phase of the transmitted signal changes by the time itis received at the receiver, the mapping to the constellation will beerrant, resulting in improper decoding. If the channel can be estimatedby the receiver, the changes in amplitude and phase can be considered bythe receiver during the mapping and decoding process.

The problem is made even more complex in V-MIMO environments. V-MIMOrelies on spatial multiplexing. In order to properly recover the signal,the receiver also must decorrelate the signals and remove interference.These tasks have traditionally been done in the time domain. These tasksare quite processing and time intensive when 2, 4 or more mobileterminals are part of the V-MIMO arrangement. The result is that thecost of equipment at the receiver becomes exorbitant, if it even can beimplemented all.

Also, while techniques for channel estimation based on least squaresalgorithms are known, these techniques are insufficient for V-MIMOimplementations, such as where two or more mobile terminal signals aresuperimposed in a set of resource blocks. Even the use of known minimummean square error (“MMSE”) techniques fall short for V-MIMOapplications.

Therefore what is needed is a cost effective, scalable and processingefficient system and method for estimating a wireless communicationchannel and cancelling interference that can be used in a V-MIMOenvironment such as on the base station uplink receiver in an LTEnetwork.

SUMMARY OF THE INVENTION

The present invention advantageously provides a method and system fordemodulating uplink data (from mobile terminal to base station) in avirtual multiple input, multiple output (“V-MIMO”) wirelesscommunication network. Reference symbol signals are used to estimate thewireless channels and the mutual interference between multiple mobileterminal or base station transmissions by using the estimates to cancelinterference from the other mobile terminals or base stationsparticipating in the V-MIMO session. These estimates are then used todemodulate the uplink user, i.e., mobile device, data signals. Error,e.g., CRC, checking is performed on the demodulated user data. In thecase where the error checking from one of the mobile terminals fails andthe error checking from the other mobile terminal passes (yielding validuser data), the correctly demodulated data from the passing mobileterminal is used to cancel the interference from the failing mobileterminal data signal. The user data signal is regeneratedpost-cancellation and the re-checked for errors.

In accordance with one aspect, the present invention provides a methodfor wireless communication in a wireless communication network in whichthe wireless communication network has a plurality of mobile terminalsarranged in virtual multiple input, multiple output (“V-MIMO”)communication with a base station. Of note, although the term V-MIMO isused herein to describe the present invention, it is understood thatthis term is not intended to limit the invention in any way and thatthis term as used herein is interchangeable with multi-user MIMO(“MU-MIMO”) and co-operative MIMO. An uplink reference signal isreceived from each of the plurality of mobile terminals. A firstreference signal channel estimate is determined for each of theplurality of mobile terminals based on the corresponding receivedreference signal. An interference cancelled estimate is received foreach of the plurality of mobile terminals using the corresponding firstreference signal channel estimate. A corresponding second referencesignal channel estimate is determined for each of the plurality ofmobile terminals based on the corresponding interference cancelledestimate.

In accordance with another aspect, the present invention provides a basestation for use in wireless communication system in which the basestation is capable of engaging in wireless communication with aplurality of mobile terminals arranged in virtual multiple inputmultiple output (“V-MIMO”) communication with the base station. The basestation receives an uplink reference signal from each of the pluralityof mobile terminals, determines a first reference signal channelestimate for each of the plurality of mobile terminals based on thecorresponding received reference signal, determines an interferencecancelled estimate for each of the plurality of mobile terminals usingthe corresponding first reference signal channel estimate and determinesa corresponding second reference signal channel estimate for each of theplurality of mobile terminals based on the corresponding interferencecancelled estimate.

In accordance with yet another aspect, the present invention provides amethod system for wireless communication in a wireless communicationnetwork. The wireless communication network has a first mobile terminaland a second mobile terminal arranged in virtual multiple input,multiple output (“V-MIMO”) communication with a base station. A firstwireless communication uplink channel corresponding to the first mobileterminal is estimated. The estimate is based on a first reference symbolsignal received from the first mobile terminal and is based on a secondreference symbol received from the second mobile terminal. The secondreference symbol signal is used to estimate and cancel the interferenceof a third reference symbol signal received from the second mobileterminal from the first reference symbol signal. A second wirelesscommunication uplink channel corresponding to the second mobile terminalis estimated. The estimate for the second wireless communication uplinkchannel is based on the third reference symbol signal received from thesecond mobile terminal and the cancellation of interference by the firstreference symbol signal received from the first mobile terminal based ona fourth reference signal received from the first mobile terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagram of an embodiment of a system constructed inaccordance with the principles of the present invention;

FIG. 2 is a block diagram of an exemplary base station constructed inaccordance with the principles of the present invention;

FIG. 3 is a block diagram of an exemplary mobile terminal constructed inaccordance with the principles of the present invention;

FIG. 4 is a block diagram of an exemplary OFDM architecture constructedin accordance with the principles of the present invention;

FIG. 5 is a block diagram of the flow of received signal processing inaccordance with the principles of the present invention;

FIG. 6 is a diagram of an exemplary scattering of pilot symbols amongavailable sub-carriers;

FIG. 7 is a block diagram of an exemplary channel estimation process ofthe present invention;

FIGS. 8A and 8B are a flow chart of an exemplary uplink datademodulation and interference cancellation process of the presentinvention;

FIG. 9 is a flow chart detailing the regeneration and the first mobileterminal to second mobile terminal interference cancellation process ofFIGS. 8A and 8B;

FIG. 10 is a flow chart detailing the regeneration and the second mobileterminal to first mobile terminal interference cancellation process ofFIGS. 8A and 8B; and

FIG. 11 is a graph of signal to noise ratio vs. symbol error rate for anumber of exemplary wireless uplink communications processes.

DETAILED DESCRIPTION OF THE INVENTION

As an initial matter, while certain embodiments are discussed in thecontext of wireless networks operating in accordance with the 3rdGeneration Partnership Project (“3GPP”) evolution, e.g., Long TermEvolution (“LTE”) standard, etc., the invention is not limited in thisregard and may be applicable to other broadband networks including thoseoperating in accordance with other orthogonal frequency divisionmultiplexing (“OFDM”)-based systems including WiMAX (IEEE 802.16) andUltra-Mobile Broadband (“UMB”), etc. Similarly, the present invention isnot limited solely to OFDM-based systems and can be implemented inaccordance with other system technologies, e.g., code division multipleaccess (“CDMA”), single carrier frequency division multiple access(“SC-FDMA”), etc.

Before describing in detail exemplary embodiments that are in accordancewith the present invention, it is noted that the embodiments resideprimarily in combinations of system components and processing stepsrelated to improving wireless communication channel estimation andinterference cancellation for virtual multiple input, multiple output(“V-MIMO”) demodulation, such as in an LTE uplink receiver.

Accordingly, the system and method components have been representedwhere appropriate by conventional symbols in the drawings, showing onlythose specific details that are pertinent to understanding theembodiments of the present invention so as not to obscure the disclosurewith details that will be readily apparent to those of ordinary skill inthe art having the benefit of the description herein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements.

Referring now to the drawing figures in which like reference designatorsrefer to like elements, there is shown in FIG. 1, a system constructedin accordance with the principles of the present invention anddesignated generally as “6.” System 6 includes one or more base stations8 (known as eNodeB in LTE systems) and one or more mobile terminals 10(shown as mobile terminals 10 a and 10 b in FIG. 1). Of note, althoughthe term “base stations” is used herein, it is understood that thesedevices are referred to as “eNodeB” devices in LTE environments.Accordingly, the use of the term “base station” herein is not intendedto limit the present invention to a particular technologyimplementation. Rather, the term “base station” is used for ease ofunderstanding, it being intended to be interchangeable with the term“eNodeB” within the context of the present invention. Although notshown, mobile terminals 10 can communicate with base stations 8 via oneor more relay nodes. Base stations 8 communicate with one another andwith external networks, such as the Internet (not shown), via carriernetwork 12. Base stations 8 engage in wireless communication with mobileterminals 10 directly or via one or more relay nodes. Similarly, mobileterminals 10 engage in wireless communication with base stations 8directly or via one or more relay nodes.

Base station 8 can be any base station arranged to wirelesslycommunicate with mobile terminals 10. Base stations 8 include thehardware and software used to implement the functions described hereinto support V-MIMO uplink channel estimation and interferencecancellation in accordance with the present invention. Base stations 8include a central processing unit, transmitter, receiver, I/O devicesand storage such as volatile and nonvolatile memory as may be needed toimplement the functions described herein. Base stations 8 are describedin additional detail below.

According to one embodiment, mobile terminals 10 may include a widerange of portable electronic devices, including but not limited tomobile phones, wireless data terminals, and similar devices, which usethe various communication technologies such as LTE, advanced mobilephone system (“AMPS”), time division multiple access (“TDMA”), CDMA,global system for mobile communications (“GSM”), general packet radioservice (“GPRS”), 1× evolution-data optimized (abbreviated as “EV-DO” or“1×EV-DO”) and universal mobile telecommunications system (“UMTS”).Mobile terminals 10 also include the hardware and software suitable tosupport the functions used to engage in wireless V-MIMO communicationwith base station 8. Such hardware can include a receiver, transmitter,central processing unit, storage in the form of volatile and nonvolatilememory, input/output devices, etc.

Relay nodes (not shown) are optionally used to facilitate wirelesscommunication between mobile terminal 10 and base station 8 in theuplink (mobile terminal 10 to base station 8) and/or the downlink (basestation 8 to mobile terminal 10). A relay node configured in accordancewith the principles of the present invention includes a centralprocessing unit, storage in the form of volatile and/or nonvolatilememory, transmitter, receiver, input/output devices and the like. Relaynodes also include software to implement the MAC control functionsdescribed herein. Of note, the arrangement shown in FIG. 1 is general innature and other specific communication embodiments constructed inaccordance with the principles of the present invention arecontemplated.

Although not shown, system 6 can include a base station controller(“BSC”) that controls wireless communications within multiple cells,which are served by corresponding base stations (′BS″) 8. It isunderstood that some implementations, such as LTE and WiMAX, do not makeuse of a BSC. In general, each base station 8 facilitates communicationsusing V-MIMO OFDM with mobile terminals 10, which are illustrated asbeing within the geographic confines of the cell 14 associated with thecorresponding base station. Movement of mobile terminals 10 in relationto the base stations 8 can result in significant fluctuation in channelconditions as a consequence of multipath distortion, terrain variation,reflection and/or interference caused by man-made objects (such asbuildings and other structures), and so on.

Multiple mobile terminals 10 may be logically grouped together to form aV-MIMO group 16. Of note, although FIG. 1 shows two mobile terminals 10grouped to form V-MIMO group 16, the invention is not limited to such.It is contemplated that more than two mobile terminals can exist in aV-MIMO group 16. It is also contemplated that a mobile terminal can havemore than one antenna to operate using traditional MIMO for wirelesscommunications as well as participate as part of a V-MIMO group 16. Evenusing diversity channels, where orthogonality-based scheduling isineffective and mobile terminals 10 therefore interfere with each other,mobile terminals 10 can still be paired in accordance with the presentinvention to take advantage of the multi-user gain associated with MIMOwireless communication.

Base station 8 is also shown in FIG. 1 as including two antennas 18 tosupport V-MIMO operation. It is understood that the present invention isnot limited to such and that base station 8 can include more than twoantennas 18 or even a single antenna 18 in support of multiple mobileterminals 10. FIG. 1 shows each mobile terminal 10 engaged in wirelesscommunication with each antenna 18 of base station 8. As is discussedbelow in detail, base station 8 includes receiver software and/orhardware to estimate the wireless channel using the minimum mean squareerror (“MMSE”) approach discussed below in detail in accordance with thepresent invention. Base station 8 also includes receiver software and/orhardware to cancel interference on the V-MIMO uplink

A high level overview of the mobile terminals 10 and base stations 8 ofthe present invention is provided prior to delving into the structuraland functional details of the preferred embodiments. It is understoodthat relay nodes can incorporate those structural and functional aspectsdescribed herein with respect to base stations 8 and mobile terminals 10as may be needed to perform the functions described herein.

With reference to FIG. 2, a base station 8 configured according to oneembodiment of the present invention is illustrated. The base station 8generally includes a control system 20, a baseband processor 22,transmit circuitry 24, receive circuitry 26, one or more antennas 18,and a network interface 30. The receive circuitry 26 receives radiofrequency signals bearing information from one or more remotetransmitters provided by mobile terminals 10 (illustrated in FIG. 3).Preferably, a low noise amplifier and a filter (not shown) cooperate toamplify and remove out-of-band interference from the signal forprocessing. Down conversion and digitization circuitry (not shown) thendown converts the filtered, received signal to an intermediate orbaseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (“DSPs”) orapplication-specific integrated circuits (“ASICs”). The receivedinformation is then sent across a wireline or wireless network via thenetwork interface 30 or transmitted to another mobile terminal 10serviced by the base station 8.

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by a carrier signal having a desiredtransmit frequency or frequencies. A power amplifier (not shown)amplifies the modulated carrier signal to a level appropriate fortransmission, and delivers the modulated carrier signal to the antennas18 through a matching network (not shown). Modulation and processingdetails are described in greater detail below.

With reference to FIG. 3, a mobile terminal 10 configured according toone embodiment of the present invention is described. Similar to basestation 8, a mobile terminal 10 constructed in accordance with theprinciples of the present invention includes a control system 32, abaseband processor 34, transmit circuitry 36, receive circuitry 38, oneor more antennas 40, and user interface circuitry 42. The receivecircuitry 38 receives radio frequency signals bearing information fromone or more base stations 8. Preferably, a low noise amplifier and afilter (not shown) cooperate to amplify and remove out-of-bandinterference from the signal for processing. Down conversion anddigitization circuitry (not shown) then down convert the filtered,received signal to an intermediate or baseband frequency signal, whichis then digitized into one or more digital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed in greater detail below. Thebaseband processor 34 is generally implemented in one or more digitalsignal processors (“DSPs”) and application specific integrated circuits(“ASICs”).

With respect to transmission, the baseband processor 34 receivesdigitized data, which may represent voice, data, or control information,from the control system 32, which the baseband processor 34 encodes fortransmission. The encoded data is output to the transmit circuitry 36,where it is used by a modulator to modulate a carrier signal that is ata desired transmit frequency or frequencies. A power amplifier (notshown) amplifies the modulated carrier signal to a level appropriate fortransmission, and delivers the modulated carrier signal to the antennas40 through a matching network (not shown). Various modulation andprocessing techniques available to those skilled in the art areapplicable to the present invention.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each carrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation is implemented, for example, through the performance ofan Inverse Fast Fourier Transform (“IFFT”) on the information to betransmitted. For demodulation, a Fast Fourier Transform (“FFT”) on thereceived signal is performed to recover the transmitted information. Inpractice, the IFFT and FFT are provided by digital signal processingcarrying out an Inverse Discrete Fourier Transform (“IDFT”) and DiscreteFourier Transform (“DFT”), respectively. Accordingly, the characterizingfeature of OFDM modulation is that orthogonal carrier waves aregenerated for multiple bands within a transmission channel. Themodulated signals are digital signals having a relatively lowtransmission rate and capable of staying within their respective bands.The individual carrier waves are not modulated directly by the digitalsignals. Instead, all carrier waves are modulated at once by IFFTprocessing.

In one embodiment, OFDM is used for at least the downlink transmissionfrom the base stations 8 to the mobile terminals 10. Each base station 8is equipped with n transmit antennas 18, and each mobile terminal 10 isequipped with one or more receive antennas 40, the total of which isreferred to as m. Notably, the respective antennas can be used forreception and transmission using appropriate duplexers or switches andare so labeled only for clarity. FIG. 1 shows n=2 and m=2.

With reference to FIG. 4, a logical OFDM transmission architecture isdescribed according to one embodiment. Initially, the base stationcontroller sends data to be transmitted to various mobile terminals 10to the base station 8. The base station 8 may use the channel qualityindicators (“CQIs”) associated with the mobile terminals to schedule thedata for transmission as well as select appropriate coding andmodulation for transmitting the scheduled data. The CQIs may be provideddirectly by the mobile terminals 10 or determined at the base station 8based on information provided by the mobile terminals 10. In eithercase, the CQI for each mobile terminal 10 is a function of the degree towhich the channel amplitude (or response) varies across the OFDMfrequency band and the strength of the transmitted signal.

The scheduled data 44, which is a stream of bits, is scrambled in amanner reducing the peak-to-average power ratio associated with the datausing data scrambling logic 46. A cyclic redundancy check (“CRC”) forthe scrambled data is determined and appended to the scrambled datausing CRC adding logic 48. Next, channel coding is performed usingchannel encoder logic 50 to effectively add redundancy to the data tofacilitate recovery and error correction at the mobile terminal 10.Again, the channel coding for a particular mobile terminal 10 is basedon the CQI. The channel encoder logic 50 uses known Turbo encodingtechniques in one embodiment. The encoded data is then processed by ratematching logic 52 to compensate for the data expansion associated withencoding.

Bit interleaver logic 54 systematically reorders the bits in the encodeddata to minimize the loss of consecutive data bits. The resultant databits are systematically mapped into corresponding symbols depending onthe chosen baseband modulation by mapping logic 56. Preferably,Quadrature Amplitude Modulation (“QAM”) or Quadrature Phase Shift Key(“QPSK”) modulation is used. The degree of modulation is preferablychosen based on the CQI for the particular mobile terminal. The symbolsmay be systematically reordered to further bolster the immunity of thetransmitted signal to periodic data loss caused by frequency selectivefading using symbol interleaver logic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. When spatialdiversity is desired, blocks of symbols are then processed by space-timeblock code (“STC”) encoder logic 60, which modifies the symbols in afashion making the transmitted signals more resistant to interferenceand more readily decoded at a mobile terminal 10. The STC encoder logic60 will process the incoming symbols and provide n outputs correspondingto the number of transmit antennas 18 for the base station 8. Thecontrol system 20 and/or baseband processor 22 will provide a mappingcontrol signal to control STC encoding. At this point, assume thesymbols for the n outputs are representative of the data to betransmitted and capable of being recovered by the mobile terminal 10.

For the present example, assume the base station 8 has two antennas 18(n=2) and the STC encoder logic 60 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 60 is sent to a corresponding IFFT processor 62,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing, alone or in combination with otherprocessing described herein. It is further envisioned that processingfunctionality can likewise be consolidated into a lesser number ofprocessors than referenced herein. The IFFT processors 62 willpreferably operate on the respective symbols to provide an inverseFourier Transform. The output of the IFFT processors 62 provides symbolsin the time domain. The time domain symbols are grouped into frames,which are associated with a prefix by like insertion logic 64. Each ofthe resultant signals is up-converted in the digital domain to anintermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 66. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 68 and antennas 18. Notably, pilotsignals known by the intended mobile terminals 10 are scattered amongthe sub-carriers. The mobile terminals 10, which are discussed in detailbelow, will use the pilot signals for channel estimation.

Reference is now made to FIG. 5 to illustrate reception of thetransmitted signals by a mobile terminal 10. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal10, the respective signals are demodulated and amplified bycorresponding RF circuitry 70. For the sake of conciseness and clarity,only one of the receive paths is described and illustrated in detail, itbeing understood that a receive path exists for each antenna 40.Analog-to-digital (“A/D”) converter and down-conversion circuitry 72digitizes and downconverts the analog signal for digital processing. Theresultant digitized signal may be used by automatic gain controlcircuitry (“AGC”) 74 to control the gain of the amplifiers in the RFcircuitry 70 based on the received signal level.

Initially, the digitized signal is provided to synchronization logic 76,which includes coarse synchronization logic 78, which buffers severalOFDM symbols and calculates an auto-correlation between the twosuccessive OFDM symbols. A resultant time index corresponding to themaximum of the correlation result determines a fine synchronizationsearch window, which is used by fine synchronization logic 80 todetermine a precise framing starting position based on the headers. Theoutput of the fine synchronization logic 80 facilitates frameacquisition by frame alignment logic 84. Proper framing alignment isimportant so that subsequent FFT processing provides an accurateconversion from the time to the frequency domain. The finesynchronization algorithm is based on the correlation between thereceived pilot signals carried by the headers and a local copy of theknown pilot data. Once frame alignment acquisition occurs, the prefix ofthe OFDM symbol is removed with prefix removal logic 86 and resultantsamples are sent to frequency offset correction logic 88, whichcompensates for the system frequency offset caused by the unmatchedlocal oscillators in the transmitter and the receiver. Preferably, thesynchronization logic 76 includes frequency offset and clock estimationlogic 82, which is based on the headers to help estimate such effects onthe transmitted signal and provide those estimations to the correctionlogic 88 to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using FFT processing logic 90. Theresults are frequency domain symbols, which are sent to processing logic92. The processing logic 92 extracts the scattered pilot signal usingscattered pilot extraction logic 94, determines a channel estimate basedon the extracted pilot signal using channel estimation logic 96, andprovides channel responses for all sub-carriers using channelreconstruction logic 98. In order to determine a channel response foreach of the sub-carriers, the pilot signal is essentially multiple pilotsymbols that are scattered among the data symbols throughout the OFDMsub-carriers in a known pattern in both time and frequency. FIG. 6illustrates an exemplary scattering of pilot symbols among availablesub-carriers over a given time and frequency plot in an OFDMenvironment. Referring again to FIG. 5, the processing logic comparesthe received pilot symbols with the pilot symbols that are expected incertain sub-carriers at certain times to determine a channel responsefor the sub-carriers in which pilot symbols were transmitted. Theresults are interpolated to estimate a channel response for most, if notall, of the remaining sub-carriers for which pilot symbols were notprovided. The actual and interpolated channel responses are used toestimate an overall channel response, which includes the channelresponses for most, if not all, of the sub-carriers in the OFDM channel.

The frequency domain symbols and channel reconstruction information,which are derived from the channel responses for each receive path areprovided to an STC decoder 100, which provides STC decoding on bothreceived paths to recover the transmitted symbols. The channelreconstruction information provides equalization information to the STCdecoder 100 sufficient to remove the effects of the transmission channelwhen processing the respective frequency domain symbols

The recovered symbols are placed back in order using symbolde-interleaver logic 102, which corresponds to the symbol interleaverlogic 58 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 104. The bits are then de-interleaved using bit de-interleaverlogic 106, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 108 and presented to channel decoder logic 110 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 112 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 114 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data 116.

Although FIGS. 4 and 5 are shown and described with respect tocommunication from base station 8 to mobile terminal 10, it isunderstood that the same concepts apply to uplink communications frommobile terminal 10 to base station 8.

The present invention provides a two part solution to the aforementionedproblems regarding channel estimation and demodulation. The first aspectprovides a reduced complexity minimum mean squared error (“MMSE”)channel estimation which allows reference signal (“RS”) channelestimation of V-MIMO mobile terminals 10. The second aspect provides fordemodulation of the data segments of the V-MIMO mobile terminals 10.Frequency domain data signal interference regeneration and cancellationis used for the weaker mobile terminal 10 signal in the V-MIMO.

Although described below in detail, the channel estimation is performedusing mutual interference cancellation from the uplink reference signalthat is jointly shared by mobile terminals 10 in the V-MIMO. This isaccomplished using sounding reference signal (“SRS”) based channelestimates. A reduced complexity MMSE method is used for RS channelestimation. With respect to data demodulation and in particular the datasignal demodulation of the weaker mobile terminal 10 in the V-MIMO, thesuccessfully decoded stronger mobile terminal 10 data signal is canceledfrom the weaker mobile terminal 10 signal and the weaker signalregenerated. This arrangement provides V-MIMO channel estimation anddemodulation performance comparable to other channel estimationtechniques, but advantageously does so at one quarter of thecomputational complexity. In other words, the present inventionadvantageously reduces the data demodulation complexity by a factor of 4compared with other demodulation methods.

The reduced complexity MMSE RS channel estimation method suitable foruse in V-MIMO RS channel estimation is described. In accordance withthis aspect of the present invention an uplink reference signal isreceived from each mobile terminal 10. Using the received uplinkreferenced signals, a first reference signal channel estimate for eachof the mobile terminals 10 is determined. For example, such a firstreference signal can be a sounding reference signal (“SRS”). The SRS foreach mobile terminal can be used to determine an interferencecancellation estimate for each mobile terminal 10. With the interferencecancellation estimate having been determined, a second reference signalchannel estimate can be determined for each of the mobile terminals 10.For example, this second reference signal channel estimate can be anMMSE channel estimate.

Of note, although the present invention is described in FIGS. 7-10 withreference to two mobile terminals 10, e.g., mobile terminal 10 a andmobile terminal 10 b, it is understood that the use of two mobileterminals is for ease of explanation and understanding only, the V-MIMOused in actual operation need not be limited to two mobile terminals 10.

The reduced complexity MMSE RS channel estimation method suitable foruse in V-MIMO RS channel estimation is described with reference to FIG.7. Initially, base station 8 receives a composite signal on the physicaluplink shared channel (“PUSCH”) from the plurality of mobile terminals10. The signal contains the received PUSCH demodulation reference symbol(“DMRS”) signal. A 1024-point Fast Fourier Transform (“FFT”) isperformed on the reference signal by base station 8 and the gainnormalized (step S100). After the FFT, and in the case of a two mobileterminal 10 V-MIMO, the signal can be described as y+x₁h₁+x₂h₂+n. Fromthe resulting frequency domain signal, the resource blocks (“RB”) can beisolated in the V-MIMO. The signal y is then normalized in step S100.With respect to this equation (y), x₁ and X₂ refer to the demodulationreference signal vectors for mobile terminal 10 a and mobile terminal 10b, respectively. h₁ and h₂ refer to the frequency domain complex channelresponse vectors of mobile terminal 10 a and mobile terminal 10 b,respectively, and n is the additive white Gaussian noise with a varianceσ_(n) ².

Because the reference symbol sequences of mobile terminals 10 a and 10 bare known to the base station 8 receiver (by virtue of being “referencesignals”), the buffered sounding reference signals (“SRS”) based channelestimates for the two mobile terminals 10 in the V-MIMO can bedetermined (steps S102 and steps S104, respectively). These estimatesare referred to herein as

{ĥ ₁} and {ĥ ₂}, respectively.

The demodulation reference signal (“DMRS”) for mobile terminals 10 a(step S106) and mobile terminal 10 b (step S108), which as discussed areknown to base station 8 are used along with the SRS estimates toregenerate an estimate of the received DMRS signal {X₁ĥ₁} for mobileterminal 10 a (step S110) and an estimate of the DMRS signal {X₂ ĥ₂} formobile terminal 10 b (step S112).

Regarding mobile terminal 10 a, equation {X₁ĥ₁} is subtracted from (y)(step S114) to create the interference canceled estimate of the DMRSsignal received from mobile terminal 10 b given by:

y _(u1) _(—) _(canceled) =y−X ₁ ĥ ₁.

The RS gain is normalized for the RS signal corresponding to mobileterminal 10 b (step S116) and a least squared (“LS”) estimation isperformed for mobile terminal 10 b (step S118) in which the LS channelestimate for mobile terminal 10 b is given by:

Ĥ _(LS2) =X ₂ ⁻¹ y _(u1) _(—) _(canceled).

A similar process is performed with respect to mobile terminal 10 a. Theregenerated signal for mobile terminal 10 b given by

{X ₂ ĥ ₂}

is subtracted from (y)(step S120) to provide an interference canceledestimate of the DMRS signal received from mobile terminal 10 a, given bythe equation

y _(u2) _(—) _(canceled) =y+X ₂ ĥ ₂.

The reference signal gain for mobile terminal 10 a is normalized (stepS122) and estimation for mobile terminal 10 a is determined (step S124).This determination is represented by:

Ĥ _(LS1) =X ₁ ⁻¹ y _(u2) _(—) _(canceled)

With the LS channel estimates having been determined, these LS estimatescan be used to determine the reduced complexity MMSE channel response toextract the DMRS channel response estimates for mobile terminals 10 aand 10 b. With respect to mobile terminal 10 a, the signal to noiseratio (“SNR”), the β, and the resource block (“RB”) length are used togenerate a correlation matrix (step S126). Channel response estimationfor mobile terminal 10 a is determined using an MMSE method (step S128).An exemplary MMSE method for step S128 is given by the followingequation:

${\hat{H}}_{{red\_ cmplx}{\_ U}\; 1} = {{R_{HH}( {R_{HH} + {\frac{\beta}{{SNR}_{1}}I}} )}^{- 1}{{\hat{H}}_{{LS}\; 1}.}}$

With respect to mobile terminal 10 b, a correlation matrix is determinedfor mobile terminal 10 b (step S130) using the SNR of mobile terminal 10b, the β for mobile terminal 10 b and the resource block length. TheMMSE channel response estimate for mobile terminal 10 b is determinedusing the LS estimation from step S118 and the MMSE correlation matrixfrom step S130 (step S132). An exemplary MMSE method for step S132 isgiven by the following equation:

${\hat{H}}_{{red\_ cmplx}{\_ U}\; 2} = {{R_{HH}( {R_{HH} + {\frac{\beta}{{SNR}_{2}}I}} )}^{- 1}{\hat{H}}_{{LS}\; 2}}$

The result of the process shown and described with reference to FIG. 7provides uplink channel estimation for mobile terminals 10 involved inV-MIMO communication with base station 8. This is done using best effortprocessing to create the effect of mobile terminal 10 a RS signal onmobile terminal 10 b and vice versa. Because base station 8 has advanceinformation and knowledge of the RS sequences of mobile terminals 10 aand 10 b, the use of frequency domain channel estimates for mobileterminals 10 a and 10 b can be used by base station 8 to approximatelygenerate each mobile terminal signal which is canceled prior to LSestimates of the other user is determined. As discussed above, thepresent invention proposes using SRS estimates. Mobile terminals in idlemode and traffic mode periodically transmit these SRS signals.Therefore, it is possible to use the SRS base channel estimates forregeneration as discussed above. For low mobility devices, e.g., mobileterminals traveling at less than 60 km/hr, the previous SRS basedchannel estimates are sufficiently accurate to generate the approximateinterference experienced from a mobile terminal 10 with respect to theother mobile terminals 10.

With the channel response estimates in hand, these estimates can be usedby base station 10 to demodulate and extract actual uplink data receivedfrom mobile terminals 10 a and 10 b. The data demodulation/extractionprocess is discussed with reference to FIGS. 8 a, 8 b, 9 and 10.

Initially base station 8 receives a PUSCH data signal from mobileterminals 10 a and 10 b. Base station 8 normalizes the data gain (stepS134) and the user data signals from mobile terminals 10 a and 10 b aredemodulated and equalized using the MMSE channel estimates from stepsS128 and S132 from FIG. 7 (steps S136 and S138, respectively). Onceequalized and demodulated, the resultant data is checked for errors,such as using a cyclic redundancy check (“CRC”). This is shown in FIG. 8a as step S140 for mobile terminal 10 a and step S142 for mobileterminal 10 b. The CRC's are then evaluated (step S144). If the CRCs forboth mobile terminal 10 a and mobile terminal 10 b pass, the demodulateddata for mobile terminals 10 a and 10 b are considered to have beenproperly demodulated and the user data are output by the process andused by base station 8 as V-MIMO mobile terminal data (step S146). Asshown in FIG. 8 a output M refers to the demodulated data from mobileterminal 10 a in step S136 and output N corresponds to the demodulateddata for mobile terminal 10 b from step S138.

If the CRC for mobile terminal 10 a passes and the CRC for mobileterminal 10 b fails (step S148), the data estimates and channelestimates of mobile terminal 10 a are used to regenerate and cancel itsinterference from mobile terminal 10 b (step S150). Step S150 isdiscussed in detail below. Because the CRC with respect to mobileterminal 10 a in step S148 passed, the user data corresponding to mobileterminal 10 a is valid and the demodulated data for mobile terminal 10 ais output by the inventive process for use by base station 8 (stepS152). This output is shown as output M. The CRC for the mobile terminal10 b user data is checked again after the interference cancellation anddata signal regeneration are applied to the signal for mobile terminal10 b (step S154). If the CRC for mobile terminal 10 b passes, thedemodulated data corresponding to mobile terminal 10 b is consideredproperly demodulated and valid and is output by the process for use bybase station 8 (step S156). The demodulated data for output mobileterminal 10 b is shown in FIG. 8 a as data U. If the CRC for mobileterminal 10 b does not pass (step S154), the next HARQ attempt formobile terminal 10 b is transmitted by base station 8 (step S158).

The case where step S148 fails, e.g., mobile terminal 10 a does not passits CRC or mobile terminal 10 b fails its CRC, a determination is madeas to whether the CRC for mobile terminal 10 a fails and the CRC mobilefor terminal 10 b passes (step S160).

In the case where the CRC has not failed for mobile terminal 10 a or theCRC terminal 10 b does not pass, e.g. where the CRC's have failed forboth mobile terminal 10 a and mobile terminal 10 b, the base station 8signals the next HARQ attempt for both mobile terminal 10 a and mobileterminal 10 b (step S162).

In the case where the CRC fails for mobile terminal 10 a and passes formobile terminal 10 b (step S160), the process of the present inventioncontinues on FIG. 8 b where the data signal and channel estimates formobile terminal 10 b are used to regenerate and cancel its interferencefrom the mobile terminal 10 a signal (step S164). Step S164 is discussedin detail below. Because the CRC corresponding to the data received frommobile device 10 b is valid, the corresponding data is valid and theprocess outputs the demodulated data wireless terminal 10 b for V-MIMOuse by base station 8 (step S166). This is shown as output N in FIG. 8B.

CRC for mobile terminal 10 a is checked again after the regeneration andinterference cancellation (step S168). If the CRC for mobile terminal 10a passes, valid data received from mobile terminal 10 a is indicated andthe process outputs the demodulated data (step S170) for use by basestation 8 to further process the V-MIMO data. The demodulated dataoutput at step S170 is shown as data V. In the case where the CRC frommobile terminal 10 a does not pass (step S168), base station 8 signalsthe next HARQ attempt for mobile terminal 10 a (step S172) and theprocess ends.

Regeneration and interference cancellation of mobile terminal 10 a withrespect to mobile 10 b of step S150 is described with reference to FIG.9. The MMSE channel estimates for mobile device 10 a (step S128) ismultiplied with the successfully demodulated data for mobile device 10 a(step S136), the resultant output of which is shown as the value M′(step S174). M′ is then subtracted from the PUSCH data for mobileterminals 10 a and 10 b (step S176). The result is the interferencecancelled and regenerated data signal for mobile terminal 10 b, shown asoutput Q.

The regeneration and cancellation of interference corresponding tomobile terminal 10 b with respect to mobile terminal 10 a of step S164is described and discussed in detail with reference to FIG. 10. Toremove the interference of mobile device 1013 from the datacorresponding to mobile device 10 a, the MMSE channel estimates formobile terminal 10 b from step S132 are multiplied with the successfullydemodulated data for mobile device 10 b from step S138 to form anestimate of the channel output based on mobile device 10 b (step S178).The resultant output is shown as N′. Of note, N′ (as well as M′) arebased on the equation discussed above given by:

y=x ₁ h ₁ +x ₂ h ₂ +n

The estimate derived as N′ is subtracted from the PUSCH data for mobileterminal 10 a and mobile terminal 10 b (step S180) to produce anestimate of the signal received from mobile device 10 a (step S180). Theresult is the interference cancelled and regenerated data signal formobile terminal 10 a, shown as output T.

FIG. 11 shows the relationship between symbol error rate and SNR formobile terminal 10 a and 10 b using quadrature phase-shift keying(“QPSK”) for mobile terminals 10 traveling at 30 km/hr and 60 km/hr.These relationships are shown with reference to the ideal channel. Theideal channel estimate shown and described with reference to FIG. 11 isdefined as having perfect knowledge of the channel, i.e., that there isno error in the estimate.

As is shown in FIG. 11 mobile terminals 10 traveling at 30 km/hr yieldsymbol error rate versus SNR curves that are less than an order ofmagnitude from the ideal estimate at low SNRs and approach the idealestimate as the SNR increases. For the faster traveling mobileterminals, the curve follows a similar path but does diverge somewhat athigher SNRs.

Of note, the graph shown in FIG. 11 is the result of a simulationassuming SRSs are transmitted once every one or two milliseconds. Thishelps to provide SNR based channel estimates that can be reliably usedfor the mutual interference cancellation in the RS signals for themobile terminals in the V-MIMO. The simulation also assumed that onemobile terminal signal is sufficiently stronger compared with the othermobile terminal signal so that the stronger mobile terminal signal datacan be reliably demodulated. This also allows the stronger mobileterminal interference to be accurately regenerated and canceled from theweaker signal and so that the interference canceled weak signal can bedemodulated. It is also assumed that the base station 8 uses a singleantenna. It is contemplated that in an embodiment using two antennas atbase station 8, turbo coding and interleaving, an additional 10 dB ofapproximate performance gain is expected for a given error rate versusSNR curve.

The present invention advantageously provides a method and system whichallows channel estimation for V-MIMO mobile terminals to be determinedin a non-computationally complex manner and also allows these channelestimates to be used to regenerate and recover mobile device uplink datathrough the interference cancellation and regeneration process discussedabove.

In implementing the present invention, it is preferable to have a highSNR mobile terminal 10 and another mobile terminal 10 with a low signalto noise ratio (“SNR”). In such case, it is likely that the high SNRmobile terminal 10 will pass its CRC, thereby providing a good source touse for the channel estimate. Pairing the high SNR and low SNR mobileterminals 10 can be done using power control. In other words, theperformance of the present invention can be enhanced by pairing upmobile terminals 10 in the V-MIMO using power control to pair up a highSNR mobile terminal 10 with a low SNR mobile terminal 10.

There are a number of other aspects of the present invention that can beimplemented to further enhance performance. For example, accommodationsof modulation coding sets (“MCS”) can be reserved for use only in V-MIMOcases. Also, in the case where the first HARQ attempt fails, subsequentHARQ attempts may be paired with other mobile terminals 10 (as comparedwith the mobile terminals 10 in the current V-MIMO set). It is alsocontemplated that uplink power control parameters of the mobileterminals 10 can be set to ensure that there is approximately a 5-10 dBdifference in their received signals. Control parameters in this casewould refer to base station 8 received signal power Po and path losscompensation factor α. It is further contemplated that V-MIMO operationcan be considered only in cases where all physical resource blocks(“PRB”) are in use and there is an incoming traffic request.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope andspirit of the invention, which is limited only by the following claims.

1-20. (canceled)
 21. A method for operating a base station as part of awireless communication network, the wireless communication networkhaving a plurality of mobile terminals arranged in virtual multipleinput, multiple output (“V-MIMO”) communication with the base station,the method comprising: receiving a composite signal in response totransmissions of respective uplink reference signals from the respectivemobile terminals, wherein the composite signal includes a superpositionof signal components corresponding respectively to the uplink referencesignals; receiving response signals in response to transmissions ofrespective sounding reference signals (SRSs) from the respective mobileterminals; determining channel estimates for the mobile terminal basedat least on the composite signal and the response signals.
 22. Themethod of claim 21, further comprising: demodulating data signalsreceived from the respective mobile terminals, wherein said demodulatingis based at least in part on the channel estimates.
 23. The method ofclaim 21, wherein said plurality of mobile terminals have been selectedto form a V-MIMO group, from a larger set of mobile terminals.
 24. Themethod of claim 21, wherein at least one of the mobile terminalsincludes a plurality of antennas.
 25. The method of claim 21, furthercomprising, for each of the plurality of mobile terminals: computing acorresponding channel response based on the corresponding channelestimate and a corresponding correlation matrix; and demodulating acorresponding received data signal using the corresponding channelresponse to obtain corresponding demodulated user data for the mobileterminal.
 26. The method of claim 21, wherein a first of the channelestimates for a first of the mobile terminals is determined by: removinga rough estimate for the signal component corresponding to a second ofthe mobile terminals from the composite signal to obtain a qualityestimate for the signal component corresponding the first mobileterminal, wherein said rough estimate is determined based on (a) theresponse signal corresponding to the second mobile terminal, (b) thesounding reference signal corresponding the second mobile terminal, and(c) the uplink reference signal corresponding the second mobileterminal; and determining said first channel estimate for the firstmobile terminal based on said quality estimate and the uplink referencesignal corresponding to the first mobile terminal.
 27. The method ofclaim 26, wherein the rough estimate for the signal componentcorresponding to the second mobile terminal is determined by: computinga preliminary channel estimate for the second mobile terminal based on(a) and (b); and computing the rough estimate for the signal componentcorresponding to the second mobile terminal based on the preliminarychannel estimate and (c).
 28. A base station for operating as part of awireless communication network, the wireless communication networkhaving a plurality of mobile terminals arranged in virtual multipleinput, multiple output (“V-MIMO”) communication with the base station,the base station comprising: a plurality of antennas; circuitry coupledto the antennas, and configured to: receive a composite signal inresponse to transmissions of respective uplink reference signals fromthe respective mobile terminals, wherein the composite signal includes asuperposition of signal components corresponding respectively to theuplink reference signals; receive response signals in response totransmissions of respective sounding reference signals (SRSs) from therespective mobile terminals; and determine channel estimates for themobile terminal based at least on the composite signal and the responsesignals.
 29. The base station of claim 28, wherein the circuitry isfurther configured to: demodulate data signals received from therespective mobile terminals, wherein said demodulating is based at leastin part on the channel estimates.
 30. The base station of claim 28,wherein the base station is configured to select said plurality ofmobile terminals from a larger set of mobile terminals, in order to forma V-MIMO group.
 31. The base station of claim 28, wherein at least oneof the mobile terminals includes a plurality of antennas.
 32. The basestation of claim 28, wherein, for each of the plurality of mobileterminals, the circuitry is further configured to: compute acorresponding channel response based on the corresponding channelestimate and a corresponding correlation matrix; and demodulate acorresponding received data signal using the corresponding channelresponse to obtain corresponding demodulated user data for the mobileterminal.
 33. The base station of claim 28, wherein the circuitry isconfigured to determine a first of the channel estimates for a first ofthe mobile terminals by: removing a rough estimate for the signalcomponent corresponding to a second of the mobile terminals from thecomposite signal to obtain a quality estimate for the signal componentcorresponding the first mobile terminal, wherein said rough estimate isdetermined based on (a) the response signal corresponding to the secondmobile terminal, (b) the sounding reference signal corresponding thesecond mobile terminal, and (c) the uplink reference signalcorresponding the second mobile terminal; and determining said firstchannel estimate for the first mobile terminal based on said qualityestimate and the uplink reference signal corresponding to the firstmobile terminal.
 34. The base station of claim 33, wherein the circuitryis configured to determine the rough estimate for the signal componentcorresponding to the second mobile terminal by: computing a preliminarychannel estimate for the second mobile terminal based on (a) and (b);and computing the rough estimate for the signal component correspondingto the second mobile terminal based on the preliminary channel estimateand (c).
 35. A non-transitory memory medium for operating a base stationas part of a wireless communication network, the wireless communicationnetwork having a plurality of mobile terminals arranged in virtualmultiple input, multiple output (“V-MIMO”) communication with the basestation, wherein the memory medium stores program instructions, whereinthe program instructions, when executed by a processor, cause the basestation to implement: receiving a composite signal in response totransmissions of respective uplink reference signals from the respectivemobile terminals, wherein the composite signal includes a superpositionof signal components corresponding respectively to the uplink referencesignals; receiving response signals in response to transmissions ofrespective sounding reference signals (SRSs) from the respective mobileterminals; and determining channel estimates for the mobile terminalbased at least on the composite signal and the response signals.
 36. Thememory medium of claim 35, wherein the program instructions, whenexecuted by the processor, further cause the base station to implement:demodulating data signals received from the respective mobile terminals,wherein said demodulating is based at least in part on the channelestimates.
 37. The memory medium of claim 35, wherein said plurality ofmobile terminals have been selected to form a V-MIMO group, from alarger set of mobile terminals.
 38. The memory medium of claim 35,wherein the program instructions, when executed by the processor,further cause the base station to implement: for each of the pluralityof mobile terminals: computing a corresponding channel response based onthe corresponding channel estimate and a corresponding correlationmatrix; and demodulating a corresponding received data signal using thecorresponding channel response to obtain corresponding demodulated userdata for the mobile terminal.
 39. The memory medium of claim 35, whereinsaid determining the channel estimates includes determining a first ofthe channel estimates for a first of the mobile terminals by: removing arough estimate for the signal component corresponding to a second of themobile terminals from the composite signal to obtain a quality estimatefor the signal component corresponding the first mobile terminal,wherein said rough estimate is determined based on (a) the responsesignal corresponding to the second mobile terminal, (b) the soundingreference signal corresponding the second mobile terminal, and (c) theuplink reference signal corresponding the second mobile terminal; anddetermining said first channel estimate for the first mobile terminalbased on said quality estimate and the uplink reference signalcorresponding to the first mobile terminal.
 40. The memory medium ofclaim 39, wherein the rough estimate for the signal componentcorresponding to the second mobile terminal is determined by: computinga preliminary channel estimate for the second mobile terminal based on(a) and (b); and computing the rough estimate for the signal componentcorresponding to the second mobile terminal based on the preliminarychannel estimate and (c).